ZX81 as a ZX80 by just changing ROM?

[mrtinb]

Since ZX80 and ZX81 are much alike. The ZX81 has added NMI/Sync.

So can I get my ZX81 to act exactly as a ZX80 by just changing the ROM?

[Paul]

Yes and no.
It does work but will need several reboot because the ZX81 ULA has no default for the startup state of the NMI generator.
The ZX81 ROM sets it off with the very first command.
The ZX80 ROM lacks this feature (because when it was designed there was no NMI-Generator).
User Holmatic of the German Forum has designed a "harness" for the CPU (four resistors if I remember right) that makes the ULA turn off the NMI-Generator before the CPU starts working.
VLA81 and Andy Rea ULAs Default to NMI OFF so these work good without the "harness"

https://www.sinclairzxworld.com/viewtop ... ness#p2837


Kind regards Paul

[Paul]

And of course the ZX81 still responds to /ROMCS when using the ZX80 ROM while the ZX80 has no /ROMCS.

[mrtinb]

Thank you for your thorough answer.

[Moggy]

I have done this with a ZX81 and can confirm it is the cheapest way to get a ZX80.

I burned an EPROM with the ZX80 image then added the resistors to the CPU and all is good.

Paul mentions a reset problem which I overcome by adding a reset switch across the reset capacitor.
If I didn't get the "K" cursor on power up a quick press of the switch always gives for me at least a perfect power up.